Experimental and Numerical Investigation of the Surface Layer Conditions after Carbonitriding of Powder Metallurgical Steels. Part 2: Surface Layer States in Components of Graded Porosity

Abstract Case hardening processes such as carbonitriding can be used to improve the performance of powder metallurgical (PM) structural components. The partially open porosity of these components leads to a significant increase in diffusion, which in turn leads to a change in the element gradient in the surface layer (and consequently the surface layer state) compared to melting metallurgic materials. Within the scope of a two-part work, the surface layer states in common densities are investigated after the case hardening process. The present part 2 comprises the characterisation of the resulting surface layer states depending on the carbon and nitrogen profile and the tempering heat treatment. Through the deep rolling and the subsequent carbonitriding treatment, hardness of up to 850 HV0.1, retained austenite contents of up to 25 vol-% and residual stresses of up to –300 MPa are determined across densities. The extensions to the numerical models described in this paper enable the FE model to predict the surface layer states in a wide range of process combinations and densities with an error tolerance of ±20 %.